1. Field of the Invention
The present invention relates to the transmission of digital signals and in particular to the demodulation of carriers in quadrature modulated by digital signals or the like.
2. Discussion of the Related Art
FIG. 1 illustrates the principle of a modulation of 4-QAM type. “QAM>> means “quadrature amplitude modulation”. In the 4-QAM modulation, two digital signals X(t) and Y(t) can each take two digital values, here represented by values −1 and +1. In FIG. 1, signal X(t) is shown on the abscissa and signal Y(t) is shown on the ordinate. Signal X(t) modulates a carrier cos(ωt) and signal Y(t) modulates a carrier in quadrature, cos(ωt+π/2). The transmitted signal S(t) is equal to:X(t). cos ωt+Y(t). cos(ωt+π/2)=X(t). cos ωt−Y(t). sin ωt.
Each couple (X,Y) in plane X,Y represents a point Mi of a constellation. In 4-QAM, the constellation is formed of four points M1, M2, M3, and M4, for example corresponding to digital values 00, 01, 11, and 10.
X(t) and Y(t) may exhibit more than two discrete levels. For example, in 16-QAM modulation, each signal X(t) and Y(t) may have four discrete levels, and the constellation comprises 16 points. The number of points in a constellation can be quite high. For example, in 2,048-QAM modulation, there are 2,048 points in the constellation.
FIG. 2 shows the well-known principle of a demodulation of signal S(t)=X(t). cos ωt−Y(t). sin ωt. The processing of signal S(t) is performed in two parallel branches. In a first branch, signal S(t) is multiplied in a multiplier 2 by a signal f1=cos(ωt). The output of multiplier 2 comprises signal X(t), in baseband, and a pulse component 2ω. The output of multiplier 2 drives a low-pass filter 4 intended to suppress pulse component 2ω. At the output of filter 4, which is assumed to be perfect, is a signal corresponding to the transmitted signal X(t).
In a second branch, said to be in quadrature, signal S(t) undergoes a multiplication by a signal f2=sin(ωt) in a multiplier 2′. The output of multiplier 2′ drives a low-pass filter 4′ intended to suppress pulse component 2ω from the signal. If the demodulation has been perfect, the output of filter 4′ provides a signal corresponding to the transmitted signal Y(t).
Generally, there exists a phase ambiguity on the carrier, and the output signals of filters 4 and 4′ must undergo a rotation in a derotator, which results in rotating the received constellation around its center 0 to have it correspond to the transmitted constellation.
Many problems are posed in the forming of demodulation circuits. Indeed, especially when frequency ω/2π of the carriers is high, as in satellite or cable transmission, where the RF frequencies of the modulated carriers are respectively on the order of approximately from 1 to 2 GHz and from 70 to 900 MHz, various errors may make the demodulators inoperative. These errors may come from different sources.
On the one hand, it is difficult to have the frequency of signals f1 and f2 used in the demodulation strictly identical to frequency ω/2π of the carriers. Indeed, due to technological dispersions or to the precision with which a frequency can be obtained, the frequency of signals f1 and f2 actually is f′=ω′/2π, with ω′ different from ω. This results in a pulse difference ω′−ω, which translates as an error in the demodulation of signal S(t).
On the other hand, it is difficult to obtain signals f1 and f2 which are strictly in phase quadrature. Thus, instead of having signals f1 and f2 respectively equal to cos ωt and sin ωt, signals f1 and f2 are equal to cosωt and sin(ωt+ε). The resulting phase quadrature error ε, which is all the more difficult to control as the variation range of frequency RF is large, introduces an additional error which must be compensated for. For example, in a signal modulator transmitted by satellite, error ε can be at most from 2 to 3 degrees. In a decoder for cable-transmitted signals, for example, by means of a 256-QAM modulation, the maximum error on phase quadrature ε must be smaller than 0.5 degrees.
A third type of error results from the gain difference existing between the two demodulation circuit branches. This error, indifferently called the gain or amplitude error, generates the same unwanted effects as quadrature and frequency errors.
FIG. 3 shows a demodulation circuit according to prior art, in which these problems are solved by the use of a digital demodulator.
In FIG. 3, a received signal RF corresponding to above signal S(t) undergoes a first frequency change in a frequency change unit 10. This frequency change is carried out by means of a variable frequency F1 enabling transposing, after filtering, frequency ω/2π of the input signal into a set intermediary frequency IF1. Frequency signal IF1 arrives on a second frequency change unit 12, driven by a frequency signal F2. Unit 12 provides an intermediary frequency signal IF2. Intermediary frequency IF2 is sufficiently low to enable sampling of the signal. For example, frequency IF2 is 36 MHz. The output signal of unit 12 arrives on an integrated circuit 14. Integrated circuit 14 includes an analog-to-digital converter 16, which converts intermediary frequency signal IF2 into a sequence of digital samples. These samples arrive on a demodulator 18. Demodulator 18 is a digital demodulator. It is driven by two signals f1 and f2 respectively equal to cos ωt and sin ωt. Due to the fact that signals f1 and f2 are digital signals, they can be obtained with the desired precision. Indeed, a phase or frequency difference between signals f1 and f2 can be simply corrected by the addition of additional bits or a modification of the bits defining these signals. Demodulator 18 provides two baseband signals X′ and Y′ which are sent to a derotator 19 providing signals X(t) and Y(t) corresponding to the transmitted signals. The output of derotator 19 forms the output of integrated circuit 14.
The circuit of FIG. 3 has the disadvantage of not being totally implementable as a single integrated circuit. Indeed, the analog-to-digital conversion of the signal before demodulation implies several frequency changes of signal RF, to lower its frequency. The frequency changes, which are analog, require very selective filters to reject unwanted image frequencies. These filters, generally surface wave filters, cannot be integrated. They are expensive and bulky. This results in many cost and bulk disadvantages. Further, the prior art solution goes against the present technological tendency, which heads towards an always-increasing integration, for example, in CMOS technology.